For more advanced QGIS users you can use OSGeo4W packages, which make it
possible to install several versions in parallel.

The OSGeo4W repository contains a lot of software from OSGeo projects.
QGIS and all dependencies are included, along with Python, GRASS, GDAL, etc.
The installer is able to install from internet or just download all needed
packages beforehand.
The downloaded files are kept in a local directory for future installations.
Steps are:

For users that cannot use OSGeo4W for some reason or just prefer it there is
also a weekly snapshot of
qgis-dev from OSGeo4W as standalone installer (taken on monday). In the
feature freeze phase that also acts as release candidate.

Don’t blindly do a full install of OSGeo4W. Only install QGIS and
maybe other components you like. Dependencies will be included
automatically. A full install pulls in components that require third party
additions, which need to be installed manually. These components render the
installation unusable without those additions.

Most linux distributions split QGIS into several packages; you’ll probably
need qgis and qgis-python (to run plugins).
Packages like qgis-grass (or qgis-plugin-grass), qgis-server can be
skipped initially, or installed only when you need them.

Below you find instructions to install per distribution. For most distro’s
there are instructions to install QGIS stable and instructions to install a
cutting edge QGIS testing build (note the warning).

To have newer versions, you have to add alternative software repositories, by
adding one of the deb-lines below to your /etc/apt/sources.list file.

Our main repository contains multiple lines of packages for several versions of
Debian and Ubuntu based on the dependencies the individual distributions
provide.

For Ubuntu we also have extra packages in a separate repository that are based
on ubuntugis (https://launchpad.net/~ubuntugis), which holds more uptodate
versions of other GIS packages than Ubuntu itself. If you want those you also
need to include ubuntugis ppa in your /etc/apt/sources.list file.

Note

The packages are only produced once shortly after a new version has
been released. As unstable, not yet released debian versions (testing) and
ubuntugis can have library changes the packages might sooner or later be
broken, when the development in debian, ubuntu or ubuntugis moves on and their
packages used as dependencies in qgis change. In that event you can either

move to a stable debian version and use the released packages for it,

wait for the next point release (due 2017-10-27 12:00:00 UTC), which
happens every month and will include the updated dependencies,

switch to the nightly repository whose packages are rebuild on regular basis
and will also pickup the updated dependencies automatically or

Latest stable openSUSE package called qgis is available for 13.1, 13.2,
Leap_42.1, Leap_42.2 and Tumbleweed (32 and 64bit). Add the following
repository to your installation manager together with the openSUSE
VERSION you work with (e.g. openSUSE_13.2).

Long Term Release package for openSUSE called qgis-ltr is available for 13.1,
13.2, Leap_42.1, Leap_42.2 and Tumbleweed (32 and 64bit). Add the following
repository to your installation manager together with the openSUSE VERSION
you work with (e.g. openSUSE_13.2).

A regularly updated development package from qgis master called qgis-master
is available for 13.1, 13.2, Leap_42.1, Leap_42.2 and Tumbleweed (32 and 64bit).
Add the following repository to your installation manager together with the
openSUSE VERSION you work with (e.g. openSUSE_13.2).

There is currently no support for Android 5. Best support is given for
Android 4.3 and 4.4.x.
This is a direct port of the QGIS desktop application. It is only slightly
optimized for touch devices and therefore needs to be carefully evaluated
for its suitability in day-to-day use. There are other apps available which
are designed and optimized specifically for touch devices.

QGIS testing packages are provided for some platforms in
addition to the QGIS stable version.
QGIS testing contains unreleased software that is currently being worked
on.
They are only provided for testing purposes to early adopters
to check if bugs have been resolved and that no new bugs have been
introduced. Although we carefully try to avoid breakages, it may at any
given time not work, or may do bad things to your data.
Take care. You have been warned!